A&A 472, 87–92 (2007) Astronomy DOI: 10.1051/0004-6361:20077513 & c ESO 2007 Astrophysics

Eight more ultra luminous X-ray candidates unmasked

C. M. Gutiérrez and M. López-Corredoira

Instituto de Astrofísica de Canarias, C/.Vía Láctea, s/n, 38200 La Laguna (S/C de Tenerife), Spain e-mail: [email protected] Received 20 March 2007 / Accepted 24 May 2007 ABSTRACT

Aims. We aim to identify the nature of several objects previously catalogued as ultra-luminous X-ray (ULX) source candidates. Methods. Long-slit spectroscopy of the optical counterparts has been carried out. Results. The eight objects are at higher than their putative parent . Conclusions. It is found that these eight previously catalogued ULX candidates, most of them projected in galactic halos are actually background unrelated objects. This result confirms the high degree of contamination present in current catalogues of ULXs. Key words. galaxies: quasars: general – X-rays: general

1. Introduction mainly background (AGN) objects existing in these compila- tions. In fact, this contamination can be on average as high Ultra-luminous X-ray sources (ULXs) are point-like objects that as 50% (López-Corredoira & Gutiérrez 2006). Similar conclu- are distributed around nearby (parent) galaxies; from the red- sions have been reached by Ptak & Colbert (2004) and Liu et al. shift of their putative parent galaxies and their X-ray flux, it is 39− 41 −1 (2006). Analyzing the spatial distribution of ULX candidates, possible to estimate their X-ray luminosity (10 10 erg s ). the contamination seems to be different according to the type of The luminosities of ULXs fill a gap between those of accret- the putative host . In fact, Irwin et al. (2004) claim that ing compact ≤1038 erg s−1, and those of active galactic nu- 43− 44 −1 elliptical galaxies do not host any ULXs with X-ray luminosi- clei (AGN; 10 10 erg s ). ULXs were discovered by the ties ≥2 × 1039 erg s−1. So, until direct confirmation of redshift, EINSTEIN satellite and were extensively found by subsequent existing compilations of such objects must be seen only as ULX X-ray missions such as ROSAT, Chandra and XMM. ULXs are candidates. This makes the identification of optical counterparts a relatively rare phenomenon; Ptak & Colbert (2004), analyz- essential. There are several reasons why these identifications are ing ROSAT data, estimated that only ∼12% of galaxies host at ffi ff di cult: some hypothetical counterparts are intrinsically faint, least one of such objects. Major e orts from the theoretical and angular resolution (in the case of ROSAT images) is poor, and observational sides have been dedicated to elucidating the still ULXs usually reside in crowded regions, thus making identifica- obscure physical nature of such sources. X-ray variability over tion of the real counterpart ambiguous. short time scales found in some objects indicates the compact In previous studies (Arp et al. 2004; Gutiérrez & López- nature of these sources (e.g., Strohmayer et al. 2007). It has been Corredoira 2005; Gutiérrez 2006), we have identified and char- proposed that ULXs could be due to super-Eddington accretion, acterized the optical counterparts of about 13 ULX candidates. supermassive X-ray binaries, or beamed emission (King et al. We showed that about 50% of the sources listed in the Colbert 2001; Körding et al. 2002). Alternatively, ULXs could repre- & Ptak catalogue (CP02 hereafter) have an optical counterpart sent the manifestation of accreting black holes of intermediate ≤20 mag. So far, nearly all of them have turned out to be back- mass (IMBHs) located in the disks or halos of nearby galaxies. ground/foreground contaminants. In general, these studies are Although there is strong evidence that relates ULXs with - biased towards the identification of objects with the brightest forming activity (e.g., Gao et al. 2003), some inactive galaxies counterparts and towards objects situated within more isolated seem to host also many ULXs (Watson et al. 2005). Some ULXs environments. ULX candidates embedded in HII regions offer are associated with recent supernovae (Roberts et al. 2003). probably more chances of being real ULXs. However, in these Observational progress in this field (and subsequent identifica- cases the optical counterpart is surrounded by the diffuse emis- tion of the most promising IMBH candidates) requires more ob- sion, thus rendering identification very difficult. In such cases servations in the X-ray region (variability studies, subarcsecond the use of Chandra and HST data is mandatory (Ptak et al. 2006). images, and spectra), and identification and characterization of Here, we extend our observational programme with the identifi- possible counterparts in other spectral domains (e.g., Abolmasov cation and characterization of eight new objects. et al. 2006; Ramsey et al. 2006). There are several major catalogues of ULX candidates by Colbert & Ptak (2002), Swartz et al. (2004), Liu & Mirabel 2. Sample selection and observations (2005) and Liu & Bregman (2005), which constitute a use- 2.1. Identification of optical counterparts ful database for conducting statistical studies or analyzing in- dividual objects. One problem that has been addressed by many We select the objects from the catalogue of ULX candidates by authors (e.g., Ptak & Colbert 2004; Irwin et al. 2004; Swartz CP02 and Swartz et al. (2004). The objects analyzed in this pa- 2006; López-Corredoira & Gutiérrez 2006) is the high degree per correspond to the optical counterparts of the sources IXO 29, of contamination by foreground (stars in our own galaxy) and 43, 54, 55, 56, 57, and 70 (we follow the same notation as that

Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20077513 88 C. M. Gutiérrez and M. López-Corredoira: Eight ULX candidates unmasked

Table 1. Optical and X-ray properties of ULXs. 1–2. main and alternative identification of the ULX candidates analyzed in this work (LM and LB stand for Liu & Mirabel 2005 and Liu & Bregman 2005, respectively); 3–4. X-ray RA (J2000) and Dec (2000) coordinates; 5. Log of the Luminosity in the 2–10 keV band (for object SW78 the quoted luminosity is in the 0.5–8 keV band) assuming that the X-ray sources were at the distance of the parent galaxy; 6–8. identification, morphological type and distance of the parent galaxy; 9. difference in arcsec between X-ray and nominal optical coordinates.

ID Alternative ID RA (J2000) Dec. (J2000) LX ID Gal. Type D ∆ (hh:mm:ss.s) (◦::)(ergs−1) (Mpc) (arcsec) IXO 29 (LM) ULX1 NGC 1961 05:41:43.3 +69:20:46 40.5 NGC 1961 +5.0 52.40 0.9 IXO 43 (LM) NGC 4151, X1; (LB) NGC4151 X6 12:10:07.9 +39:23:12 39.3 NGC 4151 +2.0 20.30 1.0 IXO 54 (LM) NGC4438 ULX1; (LB) NGC4438 X2 12:27:57.5 +13:02:30 39.9 NGC 4438 +0.0 16.80 5.2 IXO 55 (LM) NGC4472 ULX1 12:29:13.1 +07:57:40 39.1 NGC 4472 −5.0 16.80 3.7 IXO 56 (LM) NGC4472 ULX2 12:29:22.3 +07:53:31 39.4 NGC 4472 −5.0 16.80 2.7 IXO 57 (LM) NGC4472 ULX3; (LB) NGC4472 X5 12:29:23.9 +07:54:00 39.8 NGC 4472 −5.0 16.80 2.7 IXO 70 (LM) NGC4649 ULX4 12:44:07.2 +11:35:25 39.3 NGC 4649 −5.0 14.85 3.5 SW78 12:25:17.17 +18:13:46.7 39.4 NGC 4382 SA(s)0+pec 14.40 0.1

Fig. 1. DSS images centred on the putative host galaxies (left). The small black squares correspond to a one square arcmin centred on the ULX candidates analyzed in this pa- per. These regions are expanded on figures to the right. The arrows iden- tify the optical counterpart of the X- ray sources. Small circles are cen- tred on the nominal X-ray position and have radius of 5 and 1 arcsec for ROSAT and Chandra observations respectively. North is up and east to the left.

of Colbert & Ptak), and CXOU J122517.1+181346 (which we 2.2. Observations denote as SW 78 hereafter). This is the only object taken from 1 the Swartz et al. catalogue. Table 1 summarizes the main prop- The observations analyzed here were taken on the 1.93 m OHP 2 erties of such sources and those of their candidate host galaxies. and on the WHT . Table 2 presents a summary of the obser- We look for possible optical counterparts to these X-ray sources vations. The run at OHP was in March 2005; we did long-slit in the Digital Sky Survey (DSS) plates, the USNO catalogue, and the released Sloan Digital Sky Survey (SDSS) data. For the 1 These observations have been funded by the Optical Infrared eight cases considered here, we have identified a single optical Coordination network (OPTICON), a major international collaboration source compatible with the X-ray positions. The last column in supported by the Research Infrastructures Programme of the European Table 1 lists the offset between the optical and X-ray coordi- Commission Sixth Framework Programme. 2 nates. Figure 1 shows the DSS images with the identification of The Telescope (WHT) is operated by the Isaac the optical counterpart. Newton Group and the IAC in Spain’s Roque de los Muchachos Observatory. C. M. Gutiérrez and M. López-Corredoira: Eight ULX candidates unmasked 89

Table 2. Observations. 1. identification of the ULXs. 2. telescope and Table 3. Main features in the optical spectra. 1. identification of the instrument used. 3. epoch of observations. 4. exposure time. ULXs; 2. observed wavelength; 3. spectral line identification; 4. red- shift. Main ID Telescope Date Exposure time (s) ID Spectral (Å) Identification Redshift IXO 29 4.2 m WHT ISIS October 2006 3 × 1800 feature IXO 43 1.9 m OHP CARELEC March 2005 2 × 1800 IXO 29 × IXO 54 1.9 m OHP CARELEC March 2005 3 1800 5154 MgII(λ2799 Å) 0.8415 × + × IXO 55 4.2 m WHT ISIS November 2006 2 600 1 500 6866 OII(λ3728 Å) 0.8414 IXO 56 4.2 m WHT ISIS November 2006 3 × 600 λ IXO 57 1.9 m OHP CARELEC March 2005 1 × 1800 7126 NeIII( 3870 Å) 0.8415 β IXO 70 4.2 m WHT ISIS November 2006 1 × 400 8960? H 0.8426 SW78 4.2 m WHT ISIS November 2006 2 × 600 9222 OIII(λ5007 Å) 0.8413 IXO 43 4617 OII (λ3728 Å) 0.2384 optical spectroscopy with the instrument CARELEC, whose 6022 Hβ 0.2384 grating provides a resolution of 133 Å/mm. The slit was 2 arcsec. 6141 OIII(λ4959 Å) 0.2381 This instrumental configuration provided a sampling of 6203 OIII(λ5007 Å) 0.2385 IXO 54 1.78 Å and an effective resolution of 5 Å as measured from arc 4646 MgII(λ2799 Å) 0.6601 and sky lines. More details on the instrumental configuration can 6184 OII(λ3728 Å) 0.6586 be found in the telescope web pages (www.obs-hp.fr/www/ 6421 NeIII(λ3870 Å) 0.6593 guide/carelec/carelec-eng.html). With WHT we used IXO 55 the ISIS spectrograph with the grisms R300B and R158R in 3380 OIV(λ1402 Å)+SiIV(λ1397 Å) the blue and red arm respectively. The observations were car- 3735 CIV (λ1549 Å) 1.4109 ried out in October and November 2006. In November the con- 4603 CIII (λ1909 Å) 1.4114 ditions were photometric with very good seeing (∼0.5 arcsec) 6758 MgII (λ2799 Å) 1.4146 and high atmospheric transparency. The slit width was 1 arcsec. IXO 56 For wavelength calibration we took a few Cu–Ar and Cu–Ne 3529 CIII(λ1909 Å) 0.8491 lamps at the beginning and at the end of the night. The stability 4574 OII(λ2471 Å) 0.8511 of the wavelength calibration during the night was checked with 5178 MgII (λ2799 Å) 0.8503 the main sky lines. The sampling was 0.86 Å and 1.64 Å in the ff 5901 HeI(λ3189 Å)? 0.8507 blue and red arms respectively. The e ective resolution was 3.8 6890 OII(λ3728 Å) 0.8463 and 8 Å respectively. The OHP and WHT spectra were analyzed 7153 NeIII(λ3870 Å) 0.8468 3 following a standard procedure using IRAF that comprises bias IXO 57 subtraction, flat field correction, coaddition of exposures of the 5165 MgII(λ2799 Å) 0.8453 same field, extraction of the spectra and wavelength calibration. 5914? HeI(λ3189 Å)? 0.8546 We used standard spectroscopic stars from the catalogue by Oke 6910? OII(λ3728 Å) 0.8538 ff (1990) to correct for the response of the configuration to di er- 7175 NeIII(λ3870 Å) 0.8540 ent wavelengths. IXO 70 3658 CIV(λ1549 Å) 1.3628 3. Analysis 3863 HeIIλ1640 Å) 1.3548 4493 CIII(λ1909 Å) 1.3535 The spectra of the optical counterparts show features which al- 6545? MgII(λ2799 Å) 1.3397 low a clear identification and characterization. All of them, apart SW78 from IXO 43, show the presence of broad emission lines typi- 3358 CIV(λ1549 Å) 1.1674 cal of AGN/QSOs. In the case of IXO 43 the spectrum shows 4153 CIII(λ1909 Å) 1.1760 only narrow emission lines. The position of these main features 6126 MgII(λ2799 Å) 1.1887 is presented in Table 3. Analysis of each spectrum is presented below. 3.2. IXO 43 3.1. IXO 29 The X-ray source is situated close to an open arm of the Seyfert galaxy NGC 4151. There is an object at 1 arcsec from the X-ray This is one of the brightest sources in CP02. DSS plates show an position in the DSS plates that is listed in USNO with photo- object at RA = 05h41m43.4s, Dec. =+69◦2046.7 (J2000) with ∼ graphic magnitudes 18.8 and 18.1 in b and r respectively. This SNR in the peak 8 in the red plates. However this object is not object appears also in SDSS classified as a red galaxy with ugriz listed in the USNO catalogue. The spectral features identified / = magnitudes of 20.04, 19.20, 18.42, 18.06, and 17.84. This object allow us to classify the object as an AGN QSO at redshift of z was first considered by Arp (1997); it was named NGC4151: 14 0.8414, so that it is not physically associated with the putative by this author and it was found to have a redshift of z = 0.24. host galaxy NGC 1961. The real X-ray luminosity of the object However, no spectra were presented in that work. So the spec- × 44 −1 is 3.3 10 erg s , which is within typical range of values for trum presented here is the first to be published. The inferred red- AGNs. shift is z = 0.2384, a value that confirms the original estimate 3 IRAF is the Image Reduction and Analysis Facility, written and by Arp. The object, then, is not associated with NGC 4151. The supported by the IRAF programming group at the national Optical value of log OIII/Hβ = 0.33 does not allow this galaxy to be ac- Astronomy Observatories (NOAO) in Tucson, Arizona. curately classified (Veilleux & Osterbrock 1987). There are also 90 C. M. Gutiérrez and M. López-Corredoira: Eight ULX candidates unmasked

Fig. 2. Optical spectra of the counterparts of ULX sources. The y-axis is the flux in arbitrary units (counts per angstrom). Broad absorption features (obvious in most of the spectra) centered at ∼6900 and ∼7600 Å are telluric A and B bands respec- tively of molecular oxygen. some features that seem to be associated with the stellar contin- close inspection of the DSS plates reveals the presence of a faint uum (CaH & K, G-band, etc.) at the same redshift as the emis- object at SNR ∼ 3 in the peak with position RA= 12h29m12.9s, sion lines. The actual luminosity needs to be computed properly Dec. =+07◦5741.4, and then at ∼3.7 arcsec from the nom- according to the real cosmological distance. Doing that, we ob- inal ROSAT position. The object has also been detected tained an X-ray luminosity of 6.8 × 1042 erg s−1. by SDSS with magnitudes ugriz 20.57, 20.66, 20.41, 20.36 and 20.60 respectively, and with coordinates RA = 12h29m12.88s and Dec =+07◦5741.7. The spectrum is dominated by sev- 3.3. IXO 54 eral broad emission features typical of an AGN/QSO. These An optical counterpart having magnitudes of 19.2 and 17.5 in b are easily identified as CIV(λ1549 Å), CIII(λ1909 Å) and and r respectively is listed in the USNO catalogue at ∼5arcsec MgII(λ2799 Å) at 3735, 4603 and 6758 Å respectively. Another from the nominal X-ray position. No other source listed in the broad feature at 3380 Å is due to OIV(λ1402 Å) with possibly USNO is compatible with the position of the X-ray source, some contribution from SiIV (λ1397 Å). Based on these identifi- so we tentatively identify this as the optical counterpart. The cations, the redshift is z = 1.411. This corresponds to a luminos- same object has been identified in the SDSS as a stellar-type ity distance of 10 200 Mpc; the actual X-ray luminosity of the object denoted by J122757.20+130232.8 having ugriz 19.89, object is 4.6 × 1044 erg s−1. We also found the typical doublet of 19.17, 18.97, 18.85, and 18.84 respectively. The same object a strong MgII absorber at 6275 and 6291 Å, giving a redshift for is denoted in 2MASS as 12275718+1302327 and has magni- this absorption system of 1.2446. tudes of 16.762, 16.159 and 15.379 in J, H and K respec- tively. We identify the broad emission feature at ∼4646 Å as 3.5. IXO 56 MgII(λ2799 Å), and the narrow emission line at 6184 Å as We identify the optical counterpart as the source SDSS OII(λ3728 Å). The apparent feature at 5577 Å corresponds to J122922.15+075332.5. This source is not listed in USNO. residual subtraction of a sky line. Using the OII line we deter- However, as in the previous case, a faint object is located in minearedshiftofz = 0.6586. With this redshift, we identi- the DSS plates at ∼2.7 arcsec from the ROSAT position. The fied also the emission line of NeIII(λ3870 Å). The object then two most obvious spectral features are two broad emission lines turns out to be an AGN/QSO with a X-ray luminosity of 4.3 × at 3529 and 5178 Å that correspond to CIII(λ1909 Å) and − 1044 erg s 1 and an absolute magnitude (without K-correction) MgII(λ2799 Å) respectively. Based on those identifications, the = − Mr 24.07. object is an QSO/AGN with z = 0.849. We also found the narrow emission line OII(λ2471 Å) at 4574 Å. In the red 3.4. IXO 55 part of the spectrum, we tentatively identify weak emission in HeI(λ3189 Å), OII(λ3728 Å) and NeIII(λ3870 Å). With a lu- This ULX is in the halo of the elliptical NGC 4472. There are no minosity distance of 5410 Mpc, the X-ray luminosity is 2.6 × sources listed in USNO in the position of this X-ray source. A 1044 erg s−1. C. M. Gutiérrez and M. López-Corredoira: Eight ULX candidates unmasked 91

3.6. IXO 57 discovered by Knezek & Bregman (1998) as QSO1225+182 = = with a tentative redshift of z 1.19. An extended object (or An optical counterpart having r 18.7 mag is located at perhaps a couple of close objects) is located at ∼8 arcsec from 2.7 arcsec from the nominal X-ray position in the DSS plates. the optical counterpart. The spectrum of the optical counter- No other object is compatible with the X-ray position. The op- part shows three major broad emission lines that we identify as tical counterpart is identified as the star J122923.73+075359.2 CIV(λ1549 Å), CIII(λ1909 Å) and MgII(λ2799 Å) respectively. in the SDSS survey with magnitudes ugriz 19.56, 19.11, 19.04, 19.15, 19.01 respectively. During the preparation of this work4 There are two absorption lines at 3433 and 3441 Å that we iden- the SDSS DR5 was released in which the object appeared spec- tify as the signature of a strong Mg II absorber at redshift 0.2277. = From the emission lines, the object turns out to be an AGN/QSO troscopically measured as a QSO at z 0.8540. Our spectrum is = in agreement with this result. In fact, the most relevant spectral at redshift z 1.177 (this is the mean value of the ob- tained from the three main lines). The actual X-ray luminosity is feature is a broad-band emission feature centered at ∼5165 Å, − 8.2 × 1044 erg s 1. which corresponds to MgII (λ2799 Å) at z = 0.845. Based on this, we identify two possible additional narrow emission lines centered at 6910 Å and 7175 that we identify as OII(3728 Å) 4. Discussion and conclusions and NeIII(λ3870 Å). There is also some evidence for the pres- ence of CII and NeIV in the blue part of the spectrum. The dif- In the eight cases considered here, the object identified as the op- ference in redshift between the broad and the narrow features tical counterpart of the ULX candidate is the only present in DSS could be due mostly to uncertainties in the determination of the plates, the USNO catalogue or SDSS images compatible with the centroid of MgII; in any case, it would correspond to a dif- X-ray position. Liu & Bregman (2005) have determined that the − ference of ∼3000 km s 1. The X-ray luminosity of the object positional error of Rosat HRI observations are well described − is 6.6 × 1044 erg s 1. The apparent r-band magnitude (19.04) by a two-dimensional distribution with σ = 3.62 ± 0.02 arc- measured with SDSS corresponds to Mr = −24.66 (without sec. This is consistent with the differences (2.8 ± 0.6 arcsec) we K-correction). find between the position of the X-ray sources and their optical counterparts (see Table 1, last column). The positional accuracy for Chandra observations is ∼0.6 arcsec (Aldcroft et al. 2001, 3.7. IXO 70 http://cxc.harvard.edu/cal/ASPECT/celmon/) consis- The object has been observed in imaging and spectroscopy tent also with the difference between X-ray and optical positions by the SDSS collaboration, who labeled it as SDSS we found for SW78. This and the fact that all of them turn out to J124406.97+113524.3 and classify it as a QSO. The ugriz mag- be emission line galaxies (at least seven of them AGN/QSOs), nitudes are 19.48, 19.31, 19.13, 18.96 and 18.86 respectively. objects which have relative low density, gives confidence in However, according to the NED database, the determination of these identifications. In fact, a rough calculation demonstrates the redshift was uncertain, with the values z = 4.41 and 1.34 de- that a misidentification for any of the cases of the counterparts pending on the technique and lines used. There are no sources of Rosat sources that have been classified as QSO/AGNs (i.e. listed in the USNO compatible with the position of the X-ray IXO 29, 54, 55, 56, 67 and 70) is unlikely. The mean den- emission. In this case, this is rather surprising because the ob- sity of QSO/AGNs brighter than mag 20 in random fields is ject is rather conspicuous in the DSS plates having SNR ∼ 20 ∼17 per square degree (e.g. Croom et al. 2004; López-Corredoira in the peak. The optical counterpart is situated 3.5 arcsec SW of & Gutiérrez 2004), and assuming a very conservative error in the the X-ray position. We took long-slit spectra of the optical coun- X-ray nominal position of 10 arcsec, the probability to have de- − terpart with the 1.93 m OHP telescope in March 2005; however tected at least one AGN/QSO by chance is <2.5 × 10 3.Thisis the data were very noisy, making the identification of spectral certainly an upper limit to the real probability because the limit features very uncertain. We observed the object again with the in magnitude and positional errors are very conservative. Due WHT in November 2006 (these are the data presented here). The to the better resolution of Chandra observations, a misidentifi- spectrum shows a broad emission feature at 3658 Å. Two other cation in the case of SW78 is even more unlikely. The optical fainter and similar features are found at 4493 and 6545 Å. These counterpart of IXO 43 has been identified as an emission line = = allow us to classify the object as an AGN/QSO and to identify galaxy with r 18.1andb 18.8 mag in DSS plates. Although these lines as CIV(λ1549 Å), CIII(λ1909 Å) and MgII(λ2799 Å) the density of such objects is about an order of magnitude higher respectively. From the position of the CIV line (by far the most than the one of AGNs (e.g. Ho et al. 1997), a misidentification ∼ × −4 intense) we obtain a redshift z = 1.363. The X-ray luminosity is for this object is also very unlikely ( 7 10 ). then 8.6 × 1044 erg s−1. Burbidge et al. (2006) claim that the number of ULXs iden- tified as QSOs exceeds the background expectations. However, their analysis does not take into account that the total number of 3.8. SW78 galaxies surveyed (most of them without any ULX candidate) is much more larger. The density of QSOs inferred from the The object was catalogued by Swartz et al. (2004) and is the identification presented in this paper (and taken into account that only object with Chandra coordinates considered in this pa- only 12% of the galaxies explored by CP02 hosted an ULX can- per. An object coincident with the X-ray coordinates is listed didate) is roughly in agreement with the density of QSOs up to in the USNO catalogue as having magnitudes b = 18.4and b = 19 mag. Optical expected background densities and further r = 18.6. The object is well above the noise in the DSS plates discussion on possible anomalies to these densities were given and seems to be elongated. We assume this to be the opti- in López-Corredoira & Gutiérrez (2006). cal counterpart of the X-ray source. This is the same object The actual X-ray luminosities of the objects identified unam- 4 biguously as AGN/QSOs are in the narrow range 2.6×1044−8.6× Preliminary estimates of the redshift for this and some other ob- − jects analyzed in this paper were presented in advance in Gutiérrez & 1044 erg s 1, which is in the range typical for AGN/QSOs (e.g. López-Corredoira (2006) and López-Corredoira & Gutiérrez (2006). Boyle et al. 1994). The small relative separation (∼35 arcsec), 92 C. M. Gutiérrez and M. López-Corredoira: Eight ULX candidates unmasked and the similar redshifts indicate that objects IXO 56 and IXO 57 Burbidge, G., Burbidge, E. M., Arp, H. C., & Napier, W. M. 2006 are physically related, perhaps members of a larger structure. [arXiv:astro-ph/0605140] It is important to stress that these identifications are clearly Colbert, E., & Ptak, A. 2002, ApJS, 143, 25 Croom, S. M., Smith, R. J., Boyle, B. J., et al. 2004, MNRAS, 349, 1397 biased, and likely they are not representative of the nature of Gao, Y., Wang, Q. D., Appleton, P. N., & Lucas, R. A. 2003, ApJ, 596, L171 the full sample of ULX candidates. In fact, the objects analyzed Gutiérrez, C. M. 2006, ApJ, 640, L17 here belong to the subsample of ULX candidates that are rela- Gutiérrez, C. M., & López-Corredoira M. 2005, ApJ, 622, L89 tively bright in the optical (thus having a higher optical to X- Gutiérrez, C. M., & López-Corredoira M. 2006, IAUS, 230, 310 Irwin, J. A., Bregman, J. N., & Athey, A. E. 2004, ApJ, 601, L143 ray luminosity ratio). None of these belongs to the subsample Ho, L. C., Filippenko, A. V., & Sargent, W. L. W. 1997, ApJ, 487, 568 of 25 objects which López-Corredoira & Gutiérrez (2006) as- King, A. R., Davies, M. B., Ward, M. J., Fabbiano, G., & Elvis, M. 2001, ApJ, sign higher probabilities to be true ULXs. 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